There have widely been used numerical control systems for controlling a number of axes or spindles with a plurality of channels (functional arrangements). Such numerical control systems include a numerical control system for controlling a four-axis at he having two spindles and two tool rests. A workpiece can be machined at a high speed by the spindles or tool rests. With the multiple-channel numerical control system, when a single block signal is on, the machining program for a channel is interrupted after a single block, the execution of which is first finished, and the machining program for another channel is executed.
Therefore, the execution of the machining program for one channel is stopped, and the execution of the machining program for another channel is continued. As a result, the machining programs for the channels are not synchronized, the workpiece is machined out of balance, and a machining interference occurs.